Hydrostatic transmissions for work vehicles such as grass mowing machines may be used to control direction and speed of traction drive wheels. To shift between neutral and forward or reverse, the vehicle operator may move operator controls to turn a trunion shaft or control arm on the hydrostatic transmission. Turning the trunion shaft or control arm also will change the rotational speed of the wheel. When shifting the hydrostatic transmission from neutral into forward or reverse, such vehicles have a tendency to be somewhat jerky or bouncy, as the operator may not be able to smoothly control the transition from neutral. Additionally, the vehicle may experience dynamic braking, which may result in turf damage due to tire slippage and scuffing.
Hydrostatic transmissions may include a pump with a bleed passage or bypass orifice in the closed loop section of the hydraulic circuit. The bypass orifice may be included in the hydraulic circuit to help soften the abrupt transition between neutral and forward or reverse by broadening the neutral zone of the hydrostatic transmission. The bypass orifice also can provide a softer application of dynamic braking, which may reduce turf damage and enhance the vehicle's overall control, especially when mowing on a slope or hillside.
During assembly of a work vehicle with a hydrostatic transmission, a bypass orifice can help adjust and index the neutral position of the operator controls to the hydrostatic pump's exact hydraulic neutral. Hydrostatic transmissions with very small and/or no closed loop bleed passage or bypass orifice exhibit a very tight neutral or no-flow position zone. In contrast, hydrostatic transmissions equipped with larger bleed orifices have a large neutral zone, and are much easier to adjust to a no-creep position.
However, bypass orifices may have disadvantages. For example, bypass orifices may contribute to increased temperatures as loop pressures increase. Accordingly, a bypass orifice for a hydrostatic transmission is needed that provides a neutral zone sufficient to provide a smooth transition between neutral and forward or reverse, without increasing temperatures. A bypass orifice is needed that will help increase pump performance, and simplify assembly.